The present invention relates to an optical system for use with an optical memory in which information is stored or retrieved by light exposure and, more particularly, to an optical head for applying a focused laser beam onto a magneto-optical disk memory to store or retrieve information from the magneto-optical disk memory utilizing a magnetic Kerr or magnetic Faraday effect.
Magneto-optical disk memories are known which have an amphorous magnetic recording layer or film in which information is stored and from which that information may be read on the basis of the magnetic Kerr or Faraday effect. More specifically, the recording layer effectively comprises a plurality of small magnetics, the orientation of which indicates data bit information. To store such information, a laser beam is focused onto a predetermined memory track of the recording layer of a magneto-optical disk while the disk is exposed to a weak magnetic field of a predetermined direction. The laser beam forms information bits in the recording layer since the small magnet at the point of contact of the laser beam and recording layer will be aligned by the weak field, while outside that point of contact no such alignment occurs.
Information stored in a magneto-optical disk may be read from the disk directly after the information is written into the disk. More specifically, when a laser beam is focused onto the recording layer of a disk in the read mode, light is reflected by that layer. An electromagnetic vector of the resultant reflected light is rotated in correspondence with the direction of magnetization of the layer in accordance with the magnetic Kerr effect. The recorded information can thus be reproduced on the basis of the properties of this light.
Stored information can also be erased from a magneto-optical disk. More specifically, when a laser beam is focused onto the recording layer of the disk while that layer is, exposed to a magnetic field with a polarity opposite to that used in the storing mode, any information bits stored in the recording layer are effectively erased.
As noted above, light from a laser is focused onto the recording layer of a magneto-optical disk and the resultant reflected light is detected by an optical head unit. A typical example of an optical head unit for storing information in or retrieving it from a magneto-optical disk is described by Nobutaka IMAMURA in "Magneto-Optical Disk Memory", Electronics in Japan '84-85, pp.25-29.
In the optical head of the IMAMURA article, light reflected from the recording layer of a magneto-optical disk is separated into two light beams by a half mirror. On light beam is guided to a first optical system for focusing and tracking the optical head. The other light beam is guided to a second optical system that includes an analyzer for extracting a P (parallel) component or an S (Senkrecht) component from the light reflected by the magneto-optical disk. The extracted P or S component is guided to an information reading photodetector. In this example, however, the optical head includes a plurality of independent optical systems (i.e., the first and second optical systems), so the optical head configuration is complex. In addition, only one of the P and S components from the reflected light is utilized to read out the information. Therefore, the S/N ratio (Signal-to-Noise ratio) is low.
Another example of a conventional optical head for storing information in or retrieving it from a magneto-optical disk is reported by T. DEGUCHI et al., in "Digital Magneto-Optical Disk Drive", Proceedinqs of the Conference of Optical Data Storage, April 1984.
In this conventional optical head, light reflected by a magneto-optical disk is split by a beam splitter into two light beams. One light beam is guided to a first optical system for focusing and tracking the optical head. The other light beam is guided to a second optical system for data information signal read out. The second optical system includes a polarizing beam splitter for splitting the light reflected by the magneto-optical disk into P and S components. The P and S components are detected by respective Si-Avalanche photodiodes. The information signal is then reproduced in accordance with a differential readout method. The S/N ratio is higher than in the former example, but a complex optical head is still required. The increased weight of such an optical head also hinders high-speed access to a desired memory track.